1. Field of Use
This invention relates generally to winches and control means therefor. In particular, it relates to a retrieval winch which is mounted on a recovery vehicle and employed to pull an immobilized second vehicle free of unsuitable terrain.
2. Description of the Prior Art
Certain types of military tanks are extremely heavy (on the order of 70 tons) and, if maneuvered onto terrain unable to support their weight, sink in, cannot free themselves and become immobilized. In view of this problem, recovery vehicles are made available to pull the immobilized tank back onto suitable terrain. A typical recovery vehicle takes the form of a self-propelled, crawler-type, armored, manned vehicle having a retrieval winch mounted inside the recovery vehicle and also having a spade mounted on the outside of the front of the recovery vehicle. The spade, which is similar to a long narrow bulldozer blade, is lowered and dug into the earth to maintain the recovery vehicle stationary while a winch cable extending from the recovery vehicle is paid-out, attached to the immobilized tank, and then reeled in to pull the tank back onto suitable terrain. The maximum pulling force required to free the tank is very great and can exceed the weight of the tank. This force is imposed on the winch cable, on the winch and the support structure by which the winch is mounted on the recovery vehicle, and these components must be very strong mechanically.
Typically, such a retrieval winch comprises a support structure attached to the inside of the recovery vehicle, a winch drum rotatably mounted on the support structure, a winch cable having one end attached to the drum and wrapped therearound in multiple layers, a reversible winch motor mounted on the support structure for rotating the drum, a speed reduction transmission connected between the winch motor and winch drum, a normally-engaged, releasable drum brake assembly mounted on the support structure and connected to the winch drum to stop drum rotation, and a control system selectively operable to release the drum brake and operate the winch motor in the appropriate direction so as to pay-out or pull-in the winch cable. Typically, the winch motor is a single or dual displacement, reversible hydraulic motor, and the control system is hydraulic because hydraulic systems can provide high power but are relatively uncomplicated and easy to maintain and service. However, electric winch motors and control systems therefor can be employed.
A prior art retrieval winch of the aforesaid character which uses a single displacement hydraulic motor and a prior art control system therefor has an operating characteristic which necessitates that the winch components, support structure, and wire rope be designed and built so as to exert and withstand a pulling force which is substantially greater than that actually required to pull a load (i.e., free the immobilized tank) of given weight. In particular, in a single displacement hydraulic motor wherein maximum hydraulic fluid flow and pressure differential across the motor are constant, maximum motor torque and motor speed are also constant. When such a motor is connected to drive a winch on which the number of layers of cable on the winch drum increase as a cable attached to a load of given weight is pulled in, the following phenomena occurs. As the first layer of cable is formed, the first cable layer is a certain distance from the axis of drum rotation, the motor exerts a certain torque at a certain speed, and the available pulling force on the cable is at maximum and sufficient to pull the load. However, as the second layer of cable is formed, the second cable layer is at an increased distance from the axis of drum rotation, and because of this mechanical advantage (i.e., an effective lever arm of increased length), the load is "seen" by the winch motor as having increased. The winch is no longer capable of pulling the load. Thus, increased torque (above the maximum required when wrapping the first cable layer) is required from the motor. As each successive layer of cable is formed on the drum, the pulling force is proportionally decreased. As a result, the winch components must be designed to withstand the greatest force imposed thereon when a single layer of cable is formed on the drum, even though this greatest pulling force is substantially greater than the maximum force actually produced on successive cable layers. A winch designed to be capable of producing adequate cable pull on the outer most layer, must accommodate rope size and have structural integrity sufficient for the maximum line pull produced on the first layer.
As a result, a prior art retrieval winch of the aforesaid character tends to be relatively large physically because of the great pulling forces to which it is subjected and because of the manner in which the various prior art winch components are designed and arranged. It is desirable, however, that a retrieval winch installed inside of a recovery vehicle where space is at a premium be as small as practical but sufficiently powerful to do the job.